1. Field of the Invention
The present invention relates to an oxidation catalyst for cleaning exhaust gas, oxidizing particulates and hydrocarbons contained in exhaust gas from internal-combustion engines to clean the gas.
2. Description of the Related Art
For oxidizing particulates and hydrocarbons contained in exhaust gas from internal-combustion engines to clean the gas, oxidation catalysts comprising perovskite-type composite metal oxides have been previously known.
As the perovskite-type composite metal oxide used as the above-described oxidation catalyst, there is known, for example, a composite metal oxide represented by the general formula: AB1-xCxO3, wherein A is at least one metal selected from the group consisting of La, Sr, Ce, Ba, and Ca; B is at least one metal selected from the group consisting of Co, Fe, Ni, Cr, Mn, and Mg; and C is one of Pt and Pd (see Japanese Patent Laid-Open No. 07-116519).
As the perovskite-type composite metal oxide used as the above-described oxidation catalyst, there is also known, for example, a composite metal oxide represented by the general formula: CexM1-xZrO3, wherein M is at least one metal selected from the group consisting of La, Sm, Nd, Gd, Sc, and Y; and 0.1≦x≦20 (see, for example, Japanese Patent Laid-Open No. 2003-334443).
However, the above-described conventional perovskite-type composite metal oxides have drawbacks that they have high oxidation temperatures for particulates and high boiling point hydrocarbons and cannot achieve sufficient catalytic activities.